JPH11172375A - High strength bent pipe excellent in toughness in weld metal zone, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve productivity, strength, and toughness in weld zone at low temperature by providing a weld metal zone having a specific composition which consists of C, Si, Mn, P, S, Ni, Cr, Mo, Nb, Ti, Al, N, O, and the balance Fe with inevitable impurities and in which respective component values satisfy a specific relation and further having a microstructure containing specific amounts of bainite. SOLUTION: The weld metal zone has a composition consisting of, by weight, 0.03-0.10% C, <=0.6% Si, 1.5-2.2% Mn, <=0.015% P, <=0.010% S, 0.80-2.5% Ni, 0.20-1.5% Cr, 0.20-1.5% Mo, 0.01-0.10% Nb, 0.005-0.030% Ti, <=0.06% Al, 0.001-0.010% N, <=0.050% O, and the balance Fe with inevitable impurities. Moreover, the value of P defined by equation is regulated to 0.23-0.40. Further, this weld metal zone has a microstructure containing bainite of <=10 μm average grain size, transformed from austenite, by >=70% by volume fraction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to the American Petroleum Institute (A)
PI) standard X100 or more (yield strength about 690 N / mm
Bend tube (bent tube) having high strength ( ² or more) and excellent toughness of a weld metal part, and a method for producing the same.

[0002]

2. Description of the Related Art Line pipes (straight pipes) and deformed pipes (bend pipes, elbow pipes, T-shaped pipes, etc.) used in pipelines for transporting crude oil and natural gas over long distances are: (1) High pressure In order to improve the transport efficiency due to heat treatment and (2) to improve the welding efficiency in the field by reducing the thickness, the tension tends to be higher and higher. Up to now, practical application of line pipes up to X80 according to the API standard is in progress, but there is a need for higher strength line pipes and deformed pipes.

Conventionally, the mechanical properties (strength, low-temperature toughness, etc.) of a steel pipe are deteriorated in a bent pipe or the like as compared with a straight pipe.
913, JP-A-7-3330, JP-A5-
Various methods for improving the mechanical properties of a bend tube, such as 279743 and JP-A-59-232225, are disclosed.

[0004] For example, Japanese Patent Application Laid-Open No. Sho 62-10212,
JP-A-4-154913, JP-A-7-3330 and JP-A-5-279743 disclose a method in which a steel pipe is heated, quenched while being bent, and then tempered within a specific range after cooling. Is disclosed. However, these methods have a problem from the viewpoint of productivity and manufacturing cost because a tempering treatment is essential.

On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 59-232225 discloses a bend pipe for securing high strength and good low-temperature toughness by omitting a tempering treatment in order to improve productivity and reduce manufacturing costs. Are described. Japanese Patent Application Laid-Open No. 61-117223 discloses a method of manufacturing a bent pipe from a steel pipe having a weld metal of a specific component by omitting a tempering treatment. Further, JP-A 1-444
No. 769 discloses that high strength and good low-temperature toughness can be secured by quenching a low-carbon-Nb-based steel pipe while heating it while bending. However, in these methods, at most X70 (yield strength 4
90N / mm ² ) The production of bend tubes is considered to be the limit.

[0006] In order to satisfy the strength of X100 or more, it is necessary to further add an alloy element. In the above-described method, coarse upper bainite and MA (Martensite-Austenite) are contained in the structure after heating, bending, and water cooling. Constituen
t) Since a structure in which so-called martensite and austenite coexist is generated, it is considered impossible to ensure stable low-temperature toughness.

[0007] Further, due to the addition of a large amount of alloying elements, the hardness of the portion (T-cross portion) where the longitudinal welding (seam welding) of the steel pipe and the local welding at the site intersect at the time of the intermediate welding at the site of the installation is high. Problems such as cracks occur. Therefore, it has been strongly desired to develop a bend pipe having excellent productivity, high strength, and excellent weld metal toughness at low temperatures.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a bend pipe having excellent productivity, high strength and excellent weld toughness at low temperatures, and a method for producing the same.

[0009]

In order to achieve the above object, a bent pipe according to the present invention has a weld metal portion containing C:
0.03 to 0.10%, Si: 0.6% or less, Mn:
1.5 to 2.2%, P: 0.015% or less, S: 0.0
10% or less, Ni: 0.80 to 2.5%, Cr: 0.2
0 to 1.5%, Mo: 0.20 to 1.5%, Nb: 0.
01 to 0.10%, Ti: 0.005 to 0.030%,
Al: 0.06% or less, N: 0.001 to 0.010
%, O: 0.050% or less, the balance being Fe and unavoidable impurities, and a P value defined by the following formula (1) having a component composition in the range of 0.23 to 0.40; Further, it is a high-strength bend pipe excellent in toughness of a weld metal part, characterized by having a microstructure containing 70% or more in volume fraction of bainite transformed from austenite having an average particle diameter of 10 μm or less. P = C + 0.11Si + 0.03Mn + 0.02Ni + 0.04Cr + 0.07Mo + 1.46Nb (1)

[0010] Then, the metal welding portion further contains Cu: 0.1 to 1.0% and V: 0.01 to 0.10.
%, B: 0.0003 to 0.0030%, Ca: 0.0
It is preferable that the composition of the composition contains one or more of 01 to 0.005%.

Further, according to the method of the present invention, a steel pipe whose metal weld portion has the component composition described in the above-described bend pipe of the present invention can be used as a steel pipe.
This is a method for producing a high-strength bend pipe excellent in toughness of a weld metal part, characterized in that after being heated to 80 to 880 ° C., it is bent and immediately cooled with water at a cooling rate of 10 ° C./sec or more.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, it is disclosed in Japanese Patent Application Laid-Open No. 1-4769 that high strength and good low temperature toughness can be secured by quenching a low carbon-Nb steel pipe while heating it while bending. Is known by: However, in order to satisfy the strength of X100 or more, it is necessary to further add an alloy element. And simply increasing the amount of alloying elements in the weld metal deteriorates the low-temperature toughness,
The susceptibility to cracking is increased by increasing the hardness of the portion where the seam welding in the longitudinal direction of the steel pipe intersects the relay welding on site, that is, the T-cross portion increases in hardness.

Therefore, in order to secure the strength of X100 or more and the good low-temperature toughness of the weld metal part and the crack resistance of the T-cross part, a new component composition and microstructure optimization of the weld metal part are required. Is required. Then, manufacturing conditions for microstructure optimization are required.

Accordingly, as a result of diligent research for improving the low-temperature toughness of a high-strength bend pipe weld, the present invention has been achieved.
That is, the characteristics of the bend pipe of the present invention are as follows.
The system contains a limited amount of elements as described above, and
(2) The P value defined by the above equation (1) is 0.23 to 0.4
0, and (3) the microstructure of the weld metal portion contains 70% or more by volume fraction of bainite transformed from austenite having an average grain size of 10 μm or less. Such a microstructure is obtained by the method of the present invention described later. Thereby, a bend pipe having high strength of X100 or more and good low-temperature toughness of a weld metal portion can be obtained.

The low-temperature toughness of the weld metal in the longitudinal direction of the steel pipe is as follows:
It is governed by various metallurgical factors, such as (1) the size of crystal grains, and (2) the dispersed state of the hardened phase such as MA. In particular, the higher the strength, the greater the amount of alloying elements added, so that coarse upper bainite and MA, which are harmful to toughness, are formed, and the low-temperature toughness is degraded. When the upper bainite structure is formed,
It is necessary to refine crystal grains to improve low-temperature toughness.

As a result of the study by the present inventors, the average particle size was 10 μm.
If bainite is transformed from austenite of m or less, good low-temperature toughness can be obtained because the effective crystal grain size is small. At this time, unless the bainite structure contains 70% or more, the required strength of X100 or more cannot be obtained. However, as described above, even if the microstructure of the weld metal portion is strictly controlled, a steel material having desired characteristics cannot be obtained. For this purpose, it is necessary to limit the component composition simultaneously with the microstructure.

The reasons for limiting the component elements of the present invention will be described below. C is limited to 0.03 to 0.10%. C
Is an element effective for improving the strength of the base metal and the welded portion, and at least 0.03% is necessary for obtaining the desired strength in a structure mainly composed of bainite. This amount is N
It is also the minimum amount for the precipitation hardening by the addition of b and V and the manifestation of the effect of refining crystal grains. However, if the C content is too large, the low-temperature toughness and the crack resistance (on-site weldability) at the T-cross portion are significantly deteriorated, so the upper limit is set to 0.10%.

[0018] Si is an element added for deoxidation and to improve the strength, but when added in a large amount, the low-temperature toughness and on-site weldability are significantly deteriorated, so the upper limit was made 0.6%. Steel can be sufficiently deoxidized with Ti or Al, and Si need not always be added. Mn is an element indispensable for securing strength and low-temperature toughness, and its lower limit is 1.5%.
However, if the Mn content is too large, the hardenability of the steel increases and the low-temperature toughness and the on-site weldability deteriorate, so the upper limit was made 2.2%.

The purpose of adding Ni is to improve the strength of the low-carbon component system in the present invention without deteriorating low-temperature toughness or on-site weldability. Ni is added by Mn or C
Compared with the addition of r and Mo, the formation of a hardened structure harmful to the low-temperature toughness is less, and the addition of Ni is effective for improving the low-temperature toughness. To achieve this effect, 0.80%
The above addition is necessary. However, if the amount is too large,
Since not only economic efficiency but also HAZ toughness and on-site weldability are deteriorated, the upper limit is set to 2.5%.

Cr has the effect of increasing the strength of steel,
In order to exhibit this effect, it is necessary to add 0.20% or more. However, if the content is too large, the low-temperature toughness and on-site weldability are significantly deteriorated. Therefore, the upper limit of the amount of Cr is 1.5%.
And The reason for adding Mo is to improve the hardenability of steel and obtain the desired bainite-based structure. In order to obtain such an effect, Mo should be at least 0.2.
0% is required. However, excessive addition of Mo deteriorates low-temperature toughness and on-site weldability, so the upper limit was made 1.5%.

In the present invention, Nb:
0.01 to 0.10%, Ti: 0.005 to 0.030
%. Nb coexists with Mo and contributes to refinement of crystal grains and precipitation hardening, and has an effect of toughening steel. The lower limit is set to 0.
01%. However, when Nb exceeds 0.10%,
Since the low-temperature toughness and on-site weldability are adversely affected, the upper limit is set to 0.10%.

On the other hand, the addition of Ti forms fine TiN,
It suppresses coarsening of austenite grains during heating, refines the microstructure, and improves low-temperature toughness. In order to exhibit such a Ti addition effect, at least 0.005% Ti
Addition is required. However, if the amount of Ti is too large, TiN
, And precipitation hardening due to TiC occurs, and the low-temperature toughness deteriorates. Therefore, the upper limit thereof is limited to 0.030%.

Al is an element usually contained in steel as a deoxidizing material, and also has an effect on refining the structure. However, if the amount of Al exceeds 0.06%, Al-based nonmetallic inclusions increase and impair the cleanliness of the steel, so the upper limit was made 0.06%. Deoxidation can be performed with Ti or Si, and Al need not always be added.

N forms TiN and suppresses austenite grains from being coarsened during heating, and improves low-temperature toughness. The minimum required for this is 0.001%. However, if the amount is too large, it causes the deterioration of low-temperature toughness due to solid solution N, so its upper limit must be suppressed to 0.010%.

Further, in the present invention, P, which is an impurity element,
S and O contents are 0.015% or less and 0.010%, respectively.
Hereinafter, it is set to 0.050% or less. The main reason for this is to further improve the low-temperature toughness. Reduction of the P content prevents grain boundary fracture and improves low temperature toughness. Also, the reduction of the S content has the effect of improving the ductility. Reducing the amount of O is effective in reducing low-temperature toughness by reducing oxides in steel.

Next, preferable conditions are Cu, V,
The reason for adding B and Ca will be described. The main purpose of adding these elements in addition to the basic components is to maintain the excellent characteristics of the bend tube of the present invention,
This is for the purpose of increasing the manufacturable plate thickness and improving properties such as strength and toughness of the base material. Therefore, the amount of addition is of a nature that should be restricted.

The purpose of adding Cu is to improve the strength of the low carbon bend pipe of the present invention without deteriorating low-temperature toughness and on-site weldability. Cu addition is Mn, Cr,
As compared with Mo addition, a hardened structure harmful to low-temperature toughness is less formed, and addition of Cu is also effective in improving low-temperature toughness. In order to exhibit this effect, it is necessary to add 0.1% or more. However, if added in excess, precipitation hardening causes a decrease in low-temperature toughness, so the upper limit was made 1.0%.

V has almost the same effect as Nb,
The effect is weaker than Nb. However, the effect of V addition on high strength steel is significant. In order to exhibit this effect, 0.01% or more must be added. The upper limit is allowable up to 0.10% from the viewpoint of low-temperature toughness and on-site weldability.

B can drastically enhance the hardenability of steel in a trace amount to obtain a target structure mainly composed of bainite. Further, B enhances the hardenability improving effect of Mo and synergistically increases the hardenability in combination with Nb. In order to obtain such an effect, B must be at least 0.0003%. On the other hand, if it is added excessively, it not only deteriorates the low-temperature toughness but may also lose the effect of improving the hardenability of B, so the upper limit was made 0.0030%.

Ca controls the form of sulfide (MnS),
Improve low-temperature toughness (increase in absorbed energy in Charpy test, etc.). However, if the Ca content is less than 0.001%, there is no practical effect, and if the Ca content exceeds 0.005%, CaO—CaS is generated in large quantities, forming clusters and large inclusions, impairing the cleanliness of the steel. In addition, it has an adverse effect on local weldability. For this reason, the amount of Ca added was limited to 0.001 to 0.005%.

[0031] In addition to the above-mentioned limitation of the individual additive elements, the bend pipe of the present invention further includes a component composition of a weld metal portion,
The P value defined by the equation (1) is 0.23 or more and 0.40
It is necessary to limit it to the following range. This is to achieve the desired balance of strength and low-temperature toughness without impairing low-temperature toughness and on-site weldability. The reason for setting the lower limit of the P value to 0.23 is to obtain a strength of X100 or more and excellent low-temperature toughness. The upper limit of the P value is 0.40
The reason for this is to maintain excellent low-temperature toughness and on-site weldability.

Next, the method of the present invention will be described. According to the method of the present invention, it is necessary to heat a steel pipe having a weld metal part having the above-mentioned component composition to a temperature range of 780 to 880 ° C., then bend and immediately water-cool at a cooling rate of 10 ° C./second or more.

The reason why the heating temperature of the steel pipe is set to 780 ° C. or higher is to make the structure finer by partially austenitizing during heating and to obtain a predetermined strength by water cooling after bending. When the heating temperature of the steel pipe is lower than 780 ° C., austenite is formed only at the former austenite grain boundaries before heating, and when C is diffused and concentrated in this region, coarse and row-like MA is generated during water cooling after bending. Low temperature toughness deteriorates. Therefore, the lower limit of the heating temperature was set to 780 ° C.
However, when the heating temperature exceeds 880 ° C., austenite grains during heating grow, the structure after transformation becomes coarse, and the low-temperature toughness is deteriorated. Therefore, the upper limit of the heating temperature is 8
80 ° C.

After heating, the steel pipe was bent and immediately heated to 10 ° C. /
It is necessary to perform water cooling at a cooling rate of at least seconds. This is because a fine bainite structure of 70% or more is formed by cooling with water to obtain high strength and excellent low-temperature toughness. If the cooling rate is less than 10 ° C./sec, a fine bainite structure cannot be obtained, and high strength and good low-temperature toughness cannot be obtained at the same time. For this reason, the lower limit of the cooling rate during water cooling was set to 10 ° C./sec. If the steel pipe is not cooled immediately after the bending, the temperature of the steel pipe decreases, and high strength cannot be achieved due to the formation of ferrite and the like. The water cooling may be started before the completion of the bending process as long as the range does not hinder the bending process.

[0035]

EXAMPLE From a steel pipe having a constant steel composition as shown in Table 1,
Table 3 shows the results of manufacturing bend pipes having various weld metal components as shown in Table 2 and examining various properties. The mechanical properties were investigated in the direction perpendicular to the rolling. Invention Example No. 1 to
No. The weld metal part of the bend tube of the present invention No. 6 has high strength of X100 or more and good low-temperature toughness. In contrast, Comparative Example No. 7-No. The 19 bend tubes are not suitable in component composition or microstructure, and are inferior in either property.

In the present invention example No. 1 to No. No. 6 is composed of the component composition of the bend tube of the present invention and satisfies the conditions of the method of the present invention, has an appropriate microstructure, has a predetermined high strength and good low-temperature toughness. In contrast, Comparative Example No.
17-No. No. 19 is inferior in characteristics because the composition of the component is appropriate but out of the conditions of the method of the present invention.

No. Sample No. 7 has an inferior low-temperature toughness because the C content is too large. No. No. 8 has a low balance of strength and low-temperature toughness due to a small amount of Ni. No. Sample No. 9 has a low Cr content, so the strength-low temperature toughness balance is poor. No. In No. 10, the low-temperature toughness is poor because the amount of Cr is too large. No. No. 11 has low Mo toughness due to low Mo content. No. In No. 12, the low temperature toughness is poor because the Mo content is too large. No. No. 13 cannot obtain an intensity of X100 or more because the P value is small. No. No. 14 has poor low temperature toughness because the P value is too high. No. No. 15 has poor low-temperature toughness due to a large prior austenite grain size. No. No. 16 does not satisfy the strength of X100 or more because the bainite fraction is low.

No. In No. 17, since the heating temperature at the time of bending is low, the prior austenite grain size is large and the low-temperature toughness is poor. No. Sample No. 18 had a high heating temperature, so the prior austenite grain size was large, and the low-temperature toughness was poor. No. In No. 19, since the cooling rate after bending was low, the bainite fraction was low and the low-temperature toughness was poor.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

[0042]

According to the bend pipe of the present invention, the weld metal portion has an API.
Standard X100 or more (about 690N / mm ² or more in the yield strength)
And has excellent low-temperature toughness. According to the method of the present invention, such a high-strength and high-temperature tough bend pipe can be stably manufactured. As a result, the safety of the pipeline has been significantly improved, and the transportation efficiency of the pipeline has been improved.

Continued on the front page (72) Inventor Kunio Koyama 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Hitoshi Asahi 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division

Claims (3)

[Claims] C. 0.03 to 0.10%, Si: 0.6% or less, Mn: 1.5 to 2.2%, P: 0.015% by weight in a weld metal part. %: S: 0.010% or less, Ni: 0.80 to 2.5%, Cr: 0.20 to 1.5%, Mo: 0.20 to 1.5%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.030%, Al: 0.06% or less, N: 0.001 to 0.010%, O: 0.050% or less, with the balance being Fe and inevitable And the P value defined by the following equation (1) is 0.23 to 0.40
Having a microstructure containing 70% or more by volume fraction of bainite transformed from austenite having an average particle size of 10 μm or less, and having excellent toughness of a weld metal portion. High strength bend tube. P = C + 0.11Si + 0.03Mn + 0.02Ni + 0.04Cr + 0.07Mo + 1.46Nb (1) 2. The metal weld further comprises, by weight%, Cu: 0.1 to 1.0%, V: 0.01 to 0.10%, B: 0.0003 to 0.0030%, Ca: The high-strength bend pipe having excellent toughness of a weld metal part according to claim 1, characterized in that it has a component composition containing one or more of 0.001 to 0.005%. 3. A steel pipe whose metal composition has the composition described in claim 1 or 2 is heated to 780 to 880 ° C., bent, and immediately cooled with water at a cooling rate of 10 ° C./sec or more. A method for manufacturing high-strength bend pipes with excellent weld metal toughness.